Probe needle protection method for high current probe testing of power devices

ABSTRACT

A test system, apparatus and method for applying high current test stimuli to a semiconductor device in wafer or chip form includes a plurality of probes for electrically coupling to respective contact points on the semiconductor device, a plurality of current limiters electrically coupled to respective ones of the plurality of probes, and a current sensor electrically coupled to the plurality of probes. The current limiters are operative to limit current flow passing through a respective probe, and the current sensor is operative to provide a signal when detected current in any contact of the plurality of probes exceeds a threshold level.

RELATED APPLICATION DATA

This application claims the benefit of U.S. Provisional Application No.60/647,981 filed on May 23, 2006, which is incorporated herein byreference in its entirety.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to the testing of semiconductorchips, such as power semiconductor chips and more particularly, to amethod and apparatus for testing power semiconductor chips in wafer orchip form by making reliable contact to the chip via probing needles.

BACKGROUND OF THE INVENTION

Power semiconductor devices are fabricated by a series of batchprocesses in which wafers, typically of silicon or other compatiblematerial, are processed to produce a particular type of powersemiconductor chip. Each wafer contains a plurality of powersemiconductor die or chips, typically of the same kind. As is known,chemical, thermal, photolithographic and mechanical operations aretypically involved in the fabrication of the power semiconductor wafer.Because of variations across the wafer and across each individual chipcaused by process variables or physical phenomena, however, not allchips on the wafer will meet the desired specifications for the chips.Some method of testing is employed to determine which chips alone or onany given wafer meet the specifications. Since the power semiconductorsare designed to handle high currents and voltages, it is desirable totest the chip at its rated conditions. This is very difficult andpresents several unique problems when performing the tests on the waferor the chip before it is assembled into a package designed to handle thehigh power levels.

Power semiconductor chips are typically fabricated with one or morelayers of metal interconnect on the surface of the chip which provideconnecting paths to form the desired circuit. The metal interconnectlayer or layers also provide a means to make connections to the powersemiconductor chip when the chip is separated from the wafer and isassembled into a package or carrier. Interconnect points, typicallycalled “bonding pads”, are formed by the metal interconnect and arearranged on the surface of the chip so as to allow bonding wires orother connecting means to be connected from the power semiconductor chipto its carrier or package. These same bonding pads, and others designedspecifically for test purposes, can be used to make electrical contactto each individual chip for testing the electrical characteristics ofthe chips even while still joined together in wafer form or in chip formafter separation from the wafer.

The yield of good chips on a wafer is defined as the percentage of gooddies with respect to the total dies present on the wafer. Yield is thesingle most important cost factor in the production of powersemiconductor devices. Each process and test step may be considered apotential yield loss point. The testing of each die on the wafer mayresult in yield loss not only from the manufacturing processes, but alsofrom problems which can occur due to errors in testing operations. Forexample, during a probe testing operation, electrical contact is made tothe bonding pads of each power semiconductor in order to electricallystimulate the circuit and to measure critical parameters. An array offine wire probes, conductive bumps and/or fine beams formed on a cardis/are aligned so as to correspond with the array of bonding pads andis/are used to mechanically and electrically contact the array ofbonding pads. Typically, each die on the wafer is sequentiallypositioned and aligned under the array of probes, for example, and thewafer is moved up to allow contact of the respective probes onto thechip. Precision wafer movement stages allow each chip to be positionedunder the probe array, brought into contact with the probe array, andtested. The chips on the wafer which do not pass the electrical test aremarked by some method such as by applying a dot of ink or by storingtheir respective position on the wafer in computer memory for laterrecall.

In most cases, the interconnecting metal layer or layers of the powersemiconductor chip are formed of aluminum or sometimes gold. Thesemetals provide good processing characteristics and good electricalcharacteristics. However, these metals are also rather soft incomparison with the typical materials used for forming the probes on thecard (referred to herein as a power semiconductor probe card). As aresult, it is likely that damage to the bonding pad area or the probecard itself will occur if the probe card is not properly constructed,aligned, adjusted and/or utilized. For example, the tips of the probesare carefully adjusted for planarity to insure that all probes touch therespective bonding pads at relatively the same time. The probes also areadjusted to contact, e.g., touch down, accurately on each pad. After theprobes initially contact the respective bonding pads, a proper amount ofoverdrive is maintained past the point of initial contact in order toprovide a contacting force resulting in a consistent low resistancecontact.

The tips of the probes themselves should be capable of providing lowresistance contact between the probe and the bonding pads and should befree of contaminants that prevent good electrical contact. Thecontacting force or spring constant of the probe itself is also aparameter which should be considered in determining the ability of aprobe to provide a proper contact. If a probe does not make goodcontact, attempting to pass high current to die will result in excessiveheating at the contact point. This can very easily burn the tip of theprobe causing further increase in resistance and further damage to theprobe and possibly to the power semiconductor chip.

Various technologies have been used to produce probe cards for testingpower semiconductors. The most common types are blade, epoxy ring andmembrane technologies. A fourth type, which involves what is referred toa “buckling beam”, also has been used by some manufacturers. Bladetechnology is discussed in U.S. Pat. No. 4,161,692 for a “Probe Devicefor Power semiconductor Wafers”; U.S. Pat. No. 3,849,728 for a “FixedPoint Probe Card and an Assembly and Repair Fixture Therefor”; and U.S.Pat. No. 4,382,228 for a “Probes for Fixed Point Probe Cards”. Epoxyring technology is discussed in U.S. Pat. No. 3,835,381 for a “ProbeCard Including a Multiplicity of Probe Contacts and Methods of Making”;U.S. Pat. No. 3,905,008 for a “Microelectronic Test Probe Card Includinga Multiplicity of Probe Contacts and Method of Making Same”; U.S. Pat.No. 4,599,559 for “Test Probe Assembly for IC Chips”; and U.S. Pat. No.4,757,256 for a “High Density Probe Card”. Buckling beam technology isdiscussed in U.S. Pat. No. 4,554,506 for a “Modular Test Probe”; andU.S. Pat. No. 4,843,315 for a “Contact Probe Arrangement forElectrically Connecting a Test System to the Contact Pads of a Device tobe Tested”.

The most commonly used type of technology to produce power semiconductorprobe cards is epoxy ring technology, although the other technologiesare similar. In the construction of an epoxy ring type probe card, asheet of Mylar is punched or drilled with a series of holes in the samearray pattern as the bonding pad locations on the chip. The holes aresized to accept the tip of each probe and hold the tip in positionduring construction of the card. These holes are typically 0.003 inch to0.005 inch in diameter. Each probe is made from a length of spring wirewhich is tapered to a point at one end and bent down at a steep angle toform a probe tip. Each probe tip is placed in a corresponding hole inthe Mylar sheet. The other end of each spring wire probe is arrayed in agenerally circular pattern with those of the other probes and is securedin place by a ring of epoxy or another suitable material. The endsprotrude through the epoxy in order to be soldered to a circuit boardwhich forms the probe card. After the probes are soldered to the circuitboard, the probe tips are sanded to provide relatively flat probe tipspositioned in a relatively planar array.

Contact resistance of the probe tips can be measured using conventionaltechniques for the measurement of low resistances. A typical methodwould be to bring the probe tip into contact with a conducting metalsurface and measure the resistance of the resulting interface. The typeof metal used for the contact plate is typically gold, nickel or Rhodiumand some differences will be observed between the resistance measured bythese conventional methods and the actual resistance observed when theprobe is contacting bonding pads formed using aluminum metallization,for example, on the power semiconductor chip.

Furthermore, since the aluminum is rather soft in comparison with theprobe tip material, the tip of the probe will tend to protrude or “dig”into the aluminum and make contact over a much larger surface area ofthe tip as compared to on the harder gold surface. The angle of theprobe tip relative to the bonding pad is such that a scrubbing motion iscreated when the tip is driven against the pad. In the case of a bondingpad made of soft aluminum, this creates a scrub mark corresponding tothe path of the probe tip on the pad. The material used for the probetip is critical to insure a low contact resistance between the probe tipand the pad. The typical material used for power semiconductor probingis beryllium copper although some probe cards use standard tungstenprobes for lower cost.

Since a single probe can handle only a limited amount of current,multiple probe connections are typically made to each pad of the powersemiconductor chip. It is standard practice to use more probes on a padthan would typically be required to carry the desired current since itis known that some probes will have a higher resistance and not be ableto carry their share of the load. There is a trade-off between how manyprobes can be feasibly and economically placed on each pad and desiredredundancy of the connection. The typical problem with this technique isthat there is no indication of a contact problem in the test other thana loss of yield because the chip does not meet its specifications.Without some other indicator, the tester cannot determine whether it isthe chip or the probe card which is causing the problem. Testing mayproceed for some time before it can be determined that the probe contactis bad and the probe card should be serviced or replaced. Since the costof the tester, wafer prober, facility, etc., is very high, this can bevery costly and should be avoided if possible. Also, continuing to passcurrent though probes with high contact resistance will very likely burnthe probe tips and may damage the aluminum pads by creating defects inthe pad metallization.

The same conditions exist on integrated circuit chips which require highlevels of supply and ground currents such as a high performancemicroprocessor. These chips are normally constructed with multiplesupply and ground pads to share the current. As in power semiconductorchips, if some of the probe tips lose contact with their respectivepads, the remaining probe tips share the total current and operate at ahigher level. At some point this current level may become too high andexceed the specified probe tip current carrying capacity.

A second problem is that the best probes, i.e., those with the lowestresistance will tend to have the highest current. This may over stressthese “good probes” and cause them to fail thus compounding the problem.It is desirable to insure that the current is shared relatively equallyamong the probes and does not exceed the specified maximum for anysingle probe. The standard method of merely paralleling multiple probeson a given pad of a power semiconductor chip or on the multiple pads ofan integrated circuit chip does not accomplish this goal since thecontact resistance of the probe tips to the pad can vary widely.

SUMMARY OF THE INVENTION

A method and system are provided to allow high current testing of powersemiconductor chips or integrated circuit chips in wafer or chip formwhile limiting the maximum current of each individual probe to a safevalue. Further, a method and system are provided for sensing when thecontacts to the power semiconductor chip or integrated circuit chipunder test are not adequate for the current levels and removing thepower from the probes before the chip or the probe card is damaged.Additionally, a method and system are provided to sense the contactresistance of each probe during its use and predict potential problemsbefore they occur so maintenance can be scheduled before catastrophicfailure of the probe card.

According to one aspect of the invention, a test apparatus for applyinghigh current test stimuli to a semiconductor device in wafer or chipform, said semiconductor device including a plurality of contact pointsincludes: a plurality of probes for electrically coupling to respectiveones of the plurality of contact points on the semiconductor device; aplurality of current limiters electrically coupled to respective ones ofthe plurality of probes, said current limiters operative to limitcurrent flow in a corresponding probe; and a current sensor electricallycoupled to each of the plurality of probes, said current sensoroperative to provide a signal when detected current in any probe exceedsa predetermined level.

According to one aspect of the invention, the test apparatus includes aninhibit circuit operatively coupled to the current sensor, wherein whenthe current sensor generates the signal, the inhibit circuit preventscurrent flow through each of the probes.

According to one aspect of the invention, the semiconductor device is atleast one of an integrated circuit or a power semiconductor.

According to one aspect of the invention, the current limiters comprisea power transistor in combination with control circuitry, the controlcircuitry operative to limit the current flow through the powertransistor.

According to one aspect of the invention, the power transistor is apower metal oxide semiconductor field effect transistor (MOSFET) or aninsulated gate bi-polar transistor (IGBT).

According to one aspect of the invention, the current sensor comprises acurrent limit sensing circuit operative to detect when any one of thecurrent limiters transition into a constant current mode.

According to one aspect of the invention, the test apparatus includes asilicon controlled rectifier (SCR) device coupled between anemitter/source terminal and a collector/drain terminal of thesemiconductor device, wherein the current sensor comprises a currentlimit sensing circuit that drives the SCR device so as to shunt currentaway from the emitter/source and collector/drain of the semiconductordevice.

According to one aspect of the invention, the test apparatus includes averification device operative to determine if a resistance between aprobe/contact interface defined between a probe and a correspondingcontact point is within predetermined limits.

According to one aspect of the invention, the verification deviceincludes measurement circuitry to sense a voltage drop across eachindividual probe/contact interface.

According to one aspect of the invention, the measurement circuitry isoperative to inject a measurement current through each probe and tosense a resulting voltage drop across the probe/contact interface.

According to one aspect of the invention, each of the plurality ofcurrent limiters is operative to be enabled or disabled independent ofother ones of the plurality of current limiters.

According to one aspect of the invention, the current limiters areconfigured as open loop controllers.

According to one aspect of the invention, a method for applying highcurrent test stimuli to a semiconductor device in wafer or chip form,said semiconductor device including a plurality of contact points, themethod including: coupling a plurality of probes to respective ones ofthe plurality of contact points on the semiconductor device; andindependently limiting current flow through each probe of the pluralityof probes to a predetermined level.

According to one aspect of the invention, the method includes inhibitingcurrent flow through the probes upon detection of a fault condition.

According to one aspect of the invention, the method includesdetermining if a resistance between a contact interface defined betweena probe and a corresponding contact point is within predeterminedlimits.

According to one aspect of the invention, determining the resistanceincludes measuring a voltage drop across each individual contactinterface.

According to one aspect of the invention, measuring a voltage dropincludes injecting a measurement current through each probe andmeasuring the resulting voltage drop across the contact interface.

According to one aspect of the invention, independently limiting currentincludes using an open loop controller to limit the current.

According to one aspect of the invention, the method includes generatinga signal when the current flow in any probe of the plurality of probesexceeds the predetermined level.

According to one aspect of the invention, the method includes inhibitingcurrent flow through the plurality of probes upon generation of thesignal.

According to one aspect of the invention, a method of measuring acontact resistance between a plurality of probes of a test apparatus anda plurality of contact points on a semiconductor device in wafer or chipform, wherein each of the plurality of contact points correspond to afirst junction or a second junction of the semiconductor device, saidtest apparatus further including a plurality of current limiterselectrically coupled to respective ones of the plurality of probes, themethod including: enabling each of the plurality of current limiterscoupled to the first and second junctions; injecting a known currentthrough the first junction and the second junction via the plurality ofprobes; measuring a first voltage between the first junction and thesecond junction while the semiconductor device is enabled; disabling allbut one of the current limiters coupled to one of the first junction orthe second junction while leaving all of the current limiters coupled tothe other of the first junction or the second junction enabled;measuring a second voltage between the first junction and the secondjunction while the semiconductor device is enabled; and determining thecontact resistance based on the known current and the difference betweenthe first voltage and the second voltage.

To the accomplishment of the foregoing and related ends, the invention,then, comprises the features hereinafter fully described andparticularly pointed out in the claims. The following description andthe annexed drawings set forth in detail certain illustrativeembodiments of the invention. These embodiments are indicative, however,are of but a few of the various ways in which the principles of theinvention may be employed. Other objects, advantages and novel featuresof the invention will become apparent from the following detaileddescription of the invention when considered in conjunction with thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simple block diagram of an exemplary system for testingsemiconductor devices in accordance with the invention.

FIG. 2 is a schematic diagram of an exemplary power semiconductor waferincluding an exploded view of a power semiconductor chip on the wafershowing the bonding pad areas.

FIG. 3 is a cross-sectional view of an exemplary power semiconductorprobe card.

FIG. 4 is a top view of an exemplary power semiconductor chip withprobes contacting it for test.

FIG. 5 is a schematic diagram of an exemplary Unclamped InductiveSwitching (UIS) test including current limiting probe protection andcontrol in accordance with the invention.

FIG. 6 is a schematic diagram for performing an exemplary pretestmeasurement of probe tips contact resistance prior to applying high testcurrent in accordance with the invention.

FIG. 7 is a schematic diagram of an exemplary system for performinginductive switching tests on semiconductor devices, wherein the systemincludes current limiting in accordance with the invention.

FIG. 8 is a schematic diagram of an exemplary system for performingresistive switching tests on semiconductor devices, wherein the systemincludes contact resistance measurement in accordance with theinvention.

FIG. 9 is a schematic diagram of an exemplary motherboard for use withthe system of FIG. 7.

FIG. 10 is a schematic diagram of an exemplary motherboard for use withthe system of FIG. 8.

FIG. 11 is a schematic diagram of an exemplary source current limitercircuit in accordance with the invention.

FIG. 12 is a schematic diagram of an exemplary drain current limitercircuit in accordance with the invention.

FIG. 13 is a schematic diagram of an exemplary simulation circuit forsimulating the current limiters of FIGS. 10 and 11.

FIG. 14 is a graph showing inductor current versus time for thesimulation circuit of FIG. 13.

FIG. 15 is a graph showing probe current versus time for the simulationcircuit of FIG. 13.

FIG. 16 is a graph showing current limiter voltage versus time for thesimulation circuit of FIG. 13.

FIG. 17 is a graph showing contact voltage drop versus time for thesimulation circuit of FIG. 13.

FIG. 18 is a flow diagram illustrating an exemplary method ofdetermining contact resistance between probes and corresponding bondingpads in accordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described with reference to thedrawings, wherein like reference numerals are used to refer to likeelements throughout.

Referring initially to FIG. 1, a test apparatus 1 is shown for testingsemiconductor devices while in chip or wafer form. The test apparatus 1includes a plurality of probes 2 for contacting bonding pads 3 of adevice under test 4 (DUT), e.g., a semiconductor device. A test currentmay be injected into the DUT 4 via the probes 2 and bonding pads 3 so asto evaluate whether or not the DUT meets certain criteria. Each probecircuit includes a current limiter 5 that is operative to limit anamount of current flowing through the respective probe 2, independent ofthe current flowing through other probes. By limiting current flowthrough the probes, damage to the probes, DUT and/or the test apparatusitself may be prevented.

Further, a trigger circuit 6 can monitor each current limiter 5 and,upon detecting certain predefined conditions, the trigger circuit 6 mayenable SCR 7, which diverts current away from the DUT 4, therebypreventing or minimizing damage to the DUT 4 and/or probes 2.

In addition, the test apparatus 1 may be used to implement a method fordetermining if a contact resistance between the probes 2 and bondingpads 3 is within a satisfactory range. This enables possible failingprobes to be detected prior to performing test operations, therebyminimizing the likelihood of inaccurate test results.

Operation of the current limiters 5 as well as methods of using thesystem to perform pre-test measurements of contact resistance will bedescribed in more detail below.

Referring now to FIG. 2, a top view of an exemplary power semiconductorwafer 10 is shown including a plurality of power semiconductor chips 20.An enlarged view of one of the chips 20 is provided so as to illustratea plurality of bonding pads 3. It is noted that although a powersemiconductor chip is discussed herein, the device and method describedherein also may be applied to other semiconductor devices, includingintegrated circuits.

In order to determine which chips 20 on the wafer 10 meet the desiredspecifications, electrical tests are performed on each chip 20 using anexemplary power semiconductor probe card (FIG. 3). Electrical contact tothe respective chips 20 is made via the respective bonding pads 3. Thesebonding pads 3 can serve the dual purpose of providing connection pointsfor bonding wires (not shown) which provide conducting paths from thechip 20 to its package or carrier, and providing contact points forprobes to provide connection to a suitable probe card for electricallytesting the chips 20 on the wafer 10.

A side view of an exemplary probe card 45 for testing powersemiconductor chips 20 in wafer form is shown in FIG. 3. The probe card45 includes a printed circuit board 50 to which are attached a pluralityof fine wire probes 2. These probes 2 are typically attached to traceson the printed circuit board 50 by solder connections 70. The tips ofthe probes 2 are designated 90 and are arrayed in a pattern to match thebonding pad configuration of the chip 20 to be tested. The probes 2 areheld in their particular configuration by an epoxy or ceramic ring 80 asdescribed herein. The fine wire probes 2 are typically formed oftungsten, beryllium, copper or another suitable metal. The tips 90 ofthe probes 2 are typically from 0.003 inch to 0.005 inch in diameter.The tips 90 are carefully positioned to contact the bonding pads 3 (FIG.2) in such a way as to provide reliable contact to the respective pads3. The configuration of the probes 2 is such that as the probe tips 90are pressed against the bonding pads 3, some lateral motion of the tipacross the pad 3 called “scrubbing” occurs. If all of the probe tips 90were in exactly the same horizontal plane parallel to the printedcircuit board 50, this scrubbing contact action on the pads 3 would bevery predictable and repeatable. Because of the physical processesrequired to construct the probe card 45, however, the probe tips 90 willnot be perfectly planar with respect to the surface of the printedcircuit board 50 or the surface of the chip 20 on which the bonding padsarc located. If the probe tips 90 are sufficiently out of planarization(i.e., out of a planar state), the scrubbing action of the lowerpositioned probe tips 90 may cause those probe tips 90 to travel off ofthe respective pads 3 before the higher positioned probe tips 90 makecontact with the pads 3. Similarly, the probe tips 90 will not beperfectly aligned to make contact in exactly the same spot on all pads3. Misalignment of the probe tips 90 with the pads can cause the tip tomiss the pad 3 and/or possibly damage the protective oxide layer whichsurrounds the pad 3. Probe tips 90 which are sufficiently out ofplanarity may not create the force required on the pads 3 to make a goodlow resistance contact.

In order to provide a sufficiently low current path for testing the chip20, multiple probe tips 90 are positioned so as to contact the pads 3.Space considerations and voltage isolation considerations will limit thenumber of probe tips 90 and the space between the probes 2. Tradeoffsare made between a larger number of probes 2 which is desirable for lowcontact resistance and difficulty of manufacture because of the tighterspacing between the probes 2. Tighter spacing between probes 2 that havehigh voltage between them can also be a problem since a minimum spacingshould be maintained to prevent arcing between the probes 2. Anexemplary configuration of probes 2 contacting a power semiconductorchip 20 is shown in FIG. 4.

FIG. 4 shows an exemplary power semiconductor chip 20 which is typicalof a power MOSFET or IGBT. The large pads 3 are the source connectionsfor a power MOSFET or the emitter connections for an IGBT device. Thedrain connection for a power MOSFET or the collector connection for anIGBT device would be the back of the power semiconductor chip 20. Theconnection to drain or collector in FIG. 4 is represented by the testcontact area 30 which represents a typical wafer prober chuck or a metalholding plate for a single power semiconductor chip 20. Multiple probes2 a contact each source/emitter pad 3. Multiple probes 2 b contact thetest contact area 30 which is in contact with the back of the powersemiconductor chip 20 for the connection to the drain/collector. Thenumber of probes 2 is chosen such that some of the probes 2 may fail tomake contact with the pads 3 or the test contact area 30 and still notexceed the aggregate current limit of the remaining probes. As eachfailing or non-contacting probe 2 is removed from the current path, theremaining probes 2 carry additional current. Since the contactresistance of the probe tips 90 is variable, some probes 2 will carrymore current than others. At some point as more and more failing probes2 are removed from the path, the remaining probe tip 90 with the lowestcontact resistance will reach its maximum specified current. Increasingthe current through this probe 2 is likely to damage it and possiblydamage the power semiconductor chip 20 as well.

A single probe 2 is shown contacting the gate of the power semiconductorchip 20. Since there is no high current requirement for the gateconnection, a single probe is sufficient although multiple probes 2could be used for Kelvin connection or more reliable contact.

In order to prevent damage to the probe tips 90, the maximum current islimited in each probe 2. If any one probe 2 attempts to conduct morethan its maximum rated current, testing may be terminated and thecurrent diverted from the probe 2. An exemplary block diagram of asystem to accomplish this is shown in FIG. 5. A test system 1 is shownconnected to the gate 170, collector 180 and emitter 182 of an IGBTtransistor 184. For illustration purposes, the test described will beUIS (Unclamped Inductive Switching), however, the principles are thesame for any type of high current test performed on the IGBT transistor184. Each high current connection through a probe tip 90 for thecollector 180 or the emitter 182 of the IGBT 184 is limited to a definedmaximum current by a current limiter 5. Since the current levelsrequired for the test are much higher than a single probe tip 90 cancarry, multiple connections are made to the collector 180 and emitter182.

Each individual probe tip 90 connection includes a separate currentlimiter 5 to insure it never passes more than a specified maximumcurrent. During the test when current is applied, each path through aprobe tip 90 will have a different path resistance due to contactresistance of the probe tip 90, wiring resistance, etc. This will causecertain probe tips 90 with the lowest resistance to pass more currentthan others. If the path resistance of some probe tips 90 becomes veryhigh, they effectively lose contact and are not part of the currentpath. This causes the other probe tips 90 to pass more current since thetotal current is divided into the available parallel paths. Ifconditions cause a probe tip 90 to try to pass more than the maximumspecified current, the current limiter 5 enters a current source mode tolimit the current to no more than the specified value. At this point,the test may no longer be valid and should be terminated. Preferably,the test is terminated in a way which will protect the IGBT 184 undertest, the probe tips and tester 1 from damage by the high currents andvoltages. One way to accomplish this is to generate a signal 186 fromthe current limiter 5 when its programmed maximum current is reached.This signal 186 can then be used to turn on a parallel current path, inthis case an SCR 7, through a trigger circuit 6 which passes the testcurrent away from the IGBT 184. This effectively removes all highcurrents from and prevents damage to the probe tips 90.

Another method to prevent damage to the probe tips 90 is to determine bya pretest measurement that each probe tip 90 does not have excessivecontact resistance before applying the high value of test current. Thiscan be used in combination with all methods described above. Referringto FIG. 6, the measurement and control circuit 192 provides the means tomeasure the voltage drop of each individual collector 180 and emitter182 probe contact via the measurement lines 194. An emitter Kelvinmeasurement point 196 may be used to reference the measurement.Measurement of contact resistance is described in more detail below withrespect to FIG. 18.

Moving now to FIGS. 7 and 8, exemplary systems for testing semiconductordevices are shown. More specifically, FIG. 7 illustrates an exemplarytest system 200 a configured for avalanche and inductive switching (ISW)tests, while FIG. 8 illustrates an exemplary test system 200 bconfigured for resistive switching (RSW) tests. Each system 200 a and200 b includes a wafer probe system 202, a probe card motherboard 204,an IGD probe card fixture 206, a probe card 45, and a chip carrier 210.As will be described below, the respective configurations (i.e., ISW orRSW) may be implemented via the probe card motherboard 204, althoughother implementations are possible. The wafer probe system 202,motherboard 204, probe card 45 and carrier 210 are electrically coupledto one another via electrical conductors (e.g., ribbon cables, a businterface, etc.) so as to enable control signals and data signals to beexchanged therebetween.

Power may be provided to the test system 200 a and 200 b via the waferprobe system 202, which includes a DC power source 212 (e.g., a 24 VDCsource or the like) and a programmable current source 214 (e.g., a 0-10amp programmable current source). In addition, the wafer probe system202 includes safety circuits that minimize or prevent damage torespective system components. In particular, collector (drain) currentlimiters 5 b and emitter (source) current limiters 5 a are operative tolimit or clamp test current injected into the device under test (DUT) 4.The respective current limiters 5 b and 5 a will be described in moredetail below.

The wafer probe system 202 also may include a voltage measurement device222 for measuring a voltage between the collector current limiters 5 band the emitter current limiters 5 a. Preferably, the voltagemeasurement device 222 is a digital voltage sensing circuit or the like.As will be described in more detail below with respect to FIG. 18, thevoltage measurement obtained by the voltage measurement device 222 canbe used to determine a contact resistance between the probes and the DUT4.

Further, the wafer probe system 202 may include various interfaces 224that enable signals to be communicated to external equipment (e.g.,scope trigger, emitter sense, collector voltage, collector current, gatesense, etc.), as well as various communication interfaces 226 forestablishing communications between devices. These communicationinterfaces 226 may include, for example, an Ethernet interface, anRS-232 interface, an IEEE-488 interface, etc. In addition, the waferprobe system 202 may include various test points 228 accessible toexternal equipment, e.g., UIS tester 1 (FIG. 7) (e.g., to obtain voltagemeasurements at nodes of interest, etc.) and 230 (FIG. 8) (e.g., toprovide high voltage power to the system 200 b). The external equipmentthen may analyze the data and make a determination of whether or not theDUT 4 is acceptable.

The wafer probe system 202 is coupled to the motherboard 204 via ribboncables 232 or the like. In the exemplary systems 200 a and 200 b ofFIGS. 7 and 8, the motherboard 204 is used to implement the desiredtesting configuration (e.g., inductive switching or resistiveswitching). Details of the respective configurations are described belowwith respect to FIGS. 8 and 9.

In addition to being coupled to the wafer probe system 202 via theribbon cables 232, the motherboard 204 also may be coupled to the probecard 45, the chip carrier 210, and the DUT 4. For example, electricalconductors from the motherboard 204 can be connected to terminals (notshown) of the probe card 45. These terminals in turn may be coupled toprobes 2 that contact the bonding pads of a DUT 4, or are directlyconnected to the chip carrier 210.

The probe card 45 and chip carrier 210 may be mounted to or otherwisesupported by the probe card fixture 206 (e.g., a circuit board or thelike). Moreover, by housing the probe card 45 and chip carrier 210separate from the wafer probe system 202 and motherboard 204, the probecard 45 and chip carrier 210 can be moved within the work environmentwhile the wafer probe system 202 remains stationary (or substantiallystationary).

Referring now to FIG. 9, a schematic diagram of an exemplary motherboard204′ configured for inductive switching is shown. The exemplarymotherboard 204′ may be used in conjunction with the system 200 a shownin FIG. 7, for example.

The motherboard 204′ includes ribbon cable connectors 232 a′(emitter/source) 232 b′ (collector/drain) and 232 c′ (signal), eachhaving a plurality of terminals (not shown) for connecting themotherboard 204′ to the wafer probe system 202. Control signals, datasignals, and the like may be exchanged between the wafer probe system202 and the motherboard 204′ via ribbon cables 232 a-232 c andrespective connectors 232 a′-232 c′. Further, a plurality of diodesDE1-DE56 and DSE1 are connected together at their respective anodes,while an anode of zener diode 240 (also referred to as clamp 240) isconnected to the anodes of diodes DE1-DE56 and DSE1. The cathodes ofdiodes DE1-DE56 are connected to individual terminals of connector 242a, and each of these terminals may be coupled to a respective probe 2(not shown in FIG. 9) of probe card 45 (e.g., one cathode and one probeper terminal). The terminals and corresponding probes enable signals tobe provided to or obtained from the DUT 4. Further, the cathodes ofdiodes DE1-DE56 and DSE1 are coupled to respective terminals of theemitter connector 232 a′. The cathode of diode DSE1 also is coupled to afirst input terminal of a current transformer CT2.

The cathode of clamp 240 is coupled to each cathode of diodes DC1-DC40,while each anode of diodes DC1-DC40 is connected to respective terminalsof connector 242 b (one anode per terminal). As above, the each terminalof connector 242 b may be coupled to a respective probe 2 of the probecard 45 so as to provide and/or obtain signals to/from the DUT 4.Further, each anode of diode DC1-DC40 is coupled to a respectiveterminal of connector 232 b′.

A current transformer CT1 monitors the total current passing through theDUT 4, and a signal 244 from the current transformer CT1 is provided toan input of differential amplifier 246 a, while the output of thedifferential amplifier 246 a is provided to a terminal of signalconnector 232 c′. The output of differential amplifier 246 a representscollector current.

Moving back to the current transformer CT2, a second input terminal ofisolation transformer IT1 is connected to a terminal of connector 242 c,which in turn may be connected to a probe 2 (not shown in FIG. 9). Theoutput terminals of the current transformer CT2 are connected to theinput of differential amplifier 246 b, and the output of differentialamplifier 246 b is connected to a terminal of signal connector 232 c′,thereby enabling the signal to be monitored by the wafer probe system202. The output of differential amplifier 246 b represents the senseemitter current.

Further, a chip carrier sense signal and a backside collector sensesignal are obtained from the DUT 4 via respective terminals of connector242 c and probes 2 coupled thereto. These signals are provided to relayK1 (e.g., at terminals 4 and 6 of relay K1), while normally open polesof relay K1 (i.e., terminals 3 and 5) are connected to the input ofdifferential amplifier 246 c. The output of differential amplifier 246c, which represents the carrier-to-collector voltage used to computecontact resistance measurement, is connected to a terminal of signalconnector 232 c′ for monitoring by the wafer probe system 202. Bycontrolling the operation of relay K1 (e.g., by applying voltage to coilterminals 1 and 2), the output of the differential amplifier 246 c canbe toggled between zero volts and the carrier-to-collector voltage.

The backside collector sense signal along with a Kelvin emitter-2 signal(which is obtained from the DUT 4 via a terminal and probe connected toconnector 242 c) are provided to the input of differential amplifier 246d. Resistor dividers R9, R10, and R11, R12, reduce and condition theinput voltage to the differential amplifier. The output of differentialamplifier 246 d, which represents the collector voltage, is connected toa terminal of signal connector 232 c′ for monitoring by the wafer probesystem 202.

A gate sense signal, which is obtained from the DUT 4 via a probe 2 (notshown) connected to a terminal of connector 242 c, and the Kelvinemitter-2 signal are provided to an input of differential amplifier 246e. The output of differential amplifier 246 e, which represents the gatevoltage, is connected to a terminal of signal connector 232 c′ andprovided to the wafer probe system 202.

A gate driver signal obtained from the wafer probe system 202 isobtained via a terminal of connector 232 c′ and connected to an input ofgate driver 248. Further, a Kelvin emitter-1 signal is obtained from theDUT 220 via probe 2 (not shown in FIG. 9) connected to a terminal ofconnector 242 c. An output of the gate driver 248 then is provided tothe DUT 4 via a terminal of connector 242 c and a probe 2 connectedthereto. The gate driver 248 provides gating signals for the DUT 4.

The output of the gate driver 248 and a gate sense signal (which isobtained from the DUT 4 via probe 2 and connector 242 c) are provided toa gate sense Kelvin check circuit 250. Similarly, an emitter senseKelvin check circuit 252 receives both a Kelvin emitter-1 signal and theKelvin emitter-2 signal from the DUT 4 via respective probes 2 connectedto corresponding terminals of connector 242 c. Outputs of the gate senseKelvin check circuit 250 and the emitter sense Kelvin check circuit 252are provided to the wafer probe system 202 via respective terminals ofsignal connector 232 c′.

The exemplary motherboard 204′ is configured to perform unclampedswitching of the device (e.g., an avalanche test), wherein the device issubjected to industry standard ruggedness test conditions (e.g., a testwherein the breakdown voltage of the device is exceeded and the deviceis forced to absorb energy). Based on collected data, a determinationcan be made as to whether the device can absorb the energy withoutdamage. During the unclamped switching test, clamp 240 is removed fromthe circuit.

Another test performed by the exemplary motherboard 204′ is a clampedinductive switching test (e.g., a test for determining if the device canswitch a predetermined amount of current within timing limits withoutlatching on or off). During this test, the voltage seen by the device islimited so as not to exceed a predetermined level. If the voltage doesexceed the predetermined level, the energy is dissipated through theclamp 240 and diodes DE1-DE56 and DC1-DC40.

In both the clamped and unclamped tests, actual current data is obtainedfrom current transformer CT1 and provided to the wafer probe system 202via differential amplifier 246 a and signal connector 232 c′.

During the inductive switching tests, current passes through the DUT 4.Due to the inductive nature of the test, energy is stored within thesystem. When DUT 4 is the clamp will clamp the voltage across the DUT 4Note that during clamped tests, the DUT 4 will not be subjected to highvoltage spikes.

To protect the DUT 4 during the clamped test, the clamp 240 along withdiodes DE1-DE56, DC1-DC40 effectively provide a current path throughwhich the stored current may dissipate. In other words, when the clampis active, the voltage seen by the DUT 4 is limited to a predeterminedclamp value. This keeps the voltage across the DUT 4 from exceeding itsbreakdown voltage. The clamp circuit may be a passive circuit thatactivates when the voltage level across the DUT 4 exceeds its presetlevel.

During the clamped or unclamped inductive switching test, the currentthat passes through the device may be monitored relative to the gatesignals applied to the DUT (e.g., via the current transformer CT1 andthe differential amplifier circuits 246 a-246 e). The monitored currentcan be used as the switching waveform. Should the clamp 240 be activated(e.g., during a clamped inductive switching test), then the DUT currentpasses through the clamp 240 and diodes DE1-DE56 and DC1-DC40, and notthe DUT 4 or current transformer CT1.

During operation in either clamped or unclamped mode, data is collectedand provided to the wafer probe system 202 via differential amplifiercircuits 246 a-246 e. Such differential amplifier circuits are wellknown and, therefore, will not be described herein.

Moving now to the Kelvin check circuits (the gate sense Kelvin checkcircuit 250 and emitter sense Kelvin check circuit 252), these circuitseffectively provide information regarding continuity between a probe tip90 and a bonding pad 3 for the gate and emitter, respectively. Theoutput of the respective Kelvin check circuits 250 and 252 providesinformation that can be used to make sure that the respective probeshave a reasonably low resistance to the same bonding pad on the chip.

Moving now to FIG. 10, there is shown another exemplary motherboard 204″configured for resistive switching. The motherboard 204″ may be used inthe system 200 b of FIG. 8, for example. Further, the motherboard 204″includes many of the same components as the motherboard 204′ of FIG. 9.For sake of brevity, only those portions of motherboard 204″ that differfrom motherboard 204′ will be described. (emitter/source) 232 b′(collector/drain) and 232 c′ (signal), each having a plurality ofterminals (not shown) for connecting the motherboard 204′ to the waferprobe system 202 via ribbon cables 232 a-232 c or the like. A pluralityof diodes D1E1-D8E1 are connected together at their respective cathodes.The anodes of diodes D1E1-D8E1 are connected to individual terminals ofconnector 242 a, and each of these terminals may be coupled to arespective probe 2 (not shown in FIG. 10) of probe card 45 (e.g., onecathode and one probe per terminal). The terminals and correspondingprobes enable signals to be provided to or obtained from the DUT 4.Further, the anodes of diodes D1E1-D8E1 are coupled to respective anodesof diodes D1E2-D8E2, and the cathodes of diodes D1E2-D8E2 are connectedto respective terminals of the emitter connector 232 a′.

The anodes of diodes DSE1 and DSE2 are connected together and to a firstterminal of current transformer CT1. The cathode of diode DSE1 isconnected to the cathodes of diodes D1E1-D8E1, while the cathode ofdiode DSE2 is connected to a terminal of the emitter connector 232 a′.

The anodes of diodes D1C1-D8C1 are connected to respective terminals ofthe collector connector 232 b′. The cathodes of diodes D1C1-D8C1 areconnected to the cathodes of diodes D1C2-D8C2, respectively, and thecathodes of each diode pair are connected to the DUT 4 via a probe 2(not shown in FIG. 10) connected to a terminal of connector 242 b.

Each anode of diodes D1C2-D8C2 is connected to a respective resistorR1C-R8C (i.e., to a first terminal of the respective resistor). A secondterminal of each resistor is connected to a first terminal of a filtercapacitor C1, and a second terminal of the filter capacitor C1 isconnected to the cathodes of diodes D1E1-D8E1 and DSE1. A high voltagepower supply 230 includes a switch SW1, which provides power to thefirst capacitor C1.

Diodes D1C1-D8C1 and D1E1-D8E2 are operative to block reverse currentfrom the test voltages (i.e., the power supply 230) going back into thecurrent limiters 5 b and 5 a. It is noted that the current limiters 5 band 5 a are not used during resistive switching. Instead, the currentlimiters 5 b and 5 a are used only to perform measurement of contactresistance. Actual current limiting is provided by resistors R1C-R8C(which limit current to respective probes 2). The aggregate of all theresistors provides a load for the DUT 4. In other words, a deviceincludes a resistor in the drain/collector circuit and a voltage isimposed across the device, so that when the device turns on, it has aparticular current flowing through it, and when it turns off, it has thevoltage of the power supply 230 across it. Diodes D1C2-D8C2 andD1E1-D8E1 prevent the resistors from shunting across the contacts when acontact resistance measurement is performed. In other words, thesediodes block the load resistors from interfering with the contactresistance measurement.

Referring now to FIG. 11, a schematic diagram of an exemplary currentlimiter circuit is shown. The current limiter circuit is a source(emitter) current limiter circuit 5 a. A drain (collector) currentlimiter circuit 5 b will be described below with respect to FIG. 12.

The source current limiter circuit 5 a is coupled to a power source(e.g., a 10 V SOURCE REF line 300 a and a corresponding 10V SOURCE REFRETURN line 300 b). The power may be provided via an isolated DC-DCconverter, for example. A capacitor 302 or other filtering means may becoupled between the SOURCE REF line 300 a and SOURCE REF RETURN line 300b (also referred to as SOURCE COMMON 300 b), and can filter highfrequency noise, for example. Further, a voltage divider circuit 304 orthe like may be connected in parallel to the capacitor 302. The voltagedivider circuit 304 provides an adjustable voltage output as describedbelow.

The exemplary voltage divider circuit 304 includes a first resistor 306,a variable resistor 308 and a second resistor 310. A first terminal ofthe first resistor 306 is connected to the 10V SOURCE REF line 300 a,and a second terminal of the resistor 306 is connected to a firstterminal of the variable resistor 308. A second terminal of the variableresistor 308 is connected to a first terminal of the second resistor310, and a third terminal of the variable resistor 308, which is thevoltage output connection of the voltage divider 304, is coupled to afirst terminal of a capacitor 312 and to a normally closed (NC) pole ofanalog switch 314 (e.g., a single pole, double throw analog switch). Thesecond terminal of the resistor 310 and the second terminal of thecapacitor 312 are both connected to the 10V SOURCE REF RETURN line 300b. As described in more detail below, the voltage divider circuitprovides a bias voltage that can be used to turn the current limitercircuit 5 a on and off.

Terminal V+ of the analog switch 314 is connected to the 10 V SOURCE REFline 300 a, while the normally open pole (NO), the V− terminal, and theground terminal (GND) of the analog switch 314 are connected to the 10VSOURCE REF RETURN line 300 b. Terminal IN, which is coupled to theSOURCE OFF line 315, controls operation of the analog switch. Forexample, by applying or removing a control signal at the SOURCE OFF line315 (and thus terminal IN), the voltage level at the COM pole of theanalog switch 314 is toggled between SOURCE COMMON line 300 b and thevoltage divider output (i.e., the voltage across capacitor 312). Thus,each source (collector) current limiter circuit 5 b can be individuallyenabled or disabled via the SOURCE OFF line 315. An isolated eight bitport, for example, may be used to provide control signals to theindividual analog switches at each current limiter channel.

Further, the common pole (COM) of the analog switch 314 is connected tothe gate of pass device 316 (e.g., an n-channel insulated gate FET),while the source of the pass device 316 is connected to the SOURCE REFRETURN line 300 b. A capacitor 318 is connected between the gate andsource of the pass device 316. The drain of the pass device 316 isconnected to the anode of diode 320, and the cathode of diode 320 isconnected to a source limit line 322. Although not shown, the exemplarysource current limiter circuit 5 a includes sixteen source limit lines,each including a diode coupled thereto. The anode of diode 320 and drainof pass device 316 are connected to a SOURCE line 324.

The SOURCE COMMON line 300 b is bussed to all source current limiters,and connects back to the tester 1, the SCR trigger circuit 6, and thecurrent source and current measurement circuits (which may be used tomeasure contact resistance as described below). It is noted that theSOURCE COMMON line 300 b is the most negative point in the sourcecurrent limiter circuit 5 a. Further, both the source current limiters 5a and drain current limiters 5 b (described below) float relative tochassis ground.

In operation, the voltage divider 304 of the source current limitercircuit 5 a charges capacitor 312 and, once charged, the voltage isprovided to the NC pole of the analog switch 314, while the NO pole ofanalog switch 314 is coupled to the SOURCE COMMON line (e.g., at zerovolts). Based on a control signal present at the SOURCE OFF line 315,the analog switch 314 couples wither the NC pole or the NO pole to theCOM pole, which then is provided to the gate of pass device 316 (i.e.,the gate voltage is set to either the preset bias value as provided bythe voltage divider circuit 204 or to 0 volts). In other words, the passdevice is turned on or off via the SOURCE OFF line 315. As will beappreciated by those skilled in the art, the voltage divider circuit 304may be adjusted so as to properly bias the pass device 316. Capacitor318 provides a low AC impedance to hold the gate voltage of the passdevice 316 constant during test pulses despite Miller capacitancefeedback. It should also be emphasized that the current limiters shouldnot affect any Avalanche, RBSOA, Inductive Switching or even ResistiveSwitching testing.

Diode 320 feeds into a common bus that provides a diode “OR” connectionof sixteen source limiter channels. The channel having the highestvoltage between SOURCE COMMON line 300 b and corresponding SOURCE line324 will predominate, thereby turning the diode 320 on. This signal thenis compared to a reference to determine if excessive voltage existsacross any of the current limiters. If so, the SCR trigger circuit 6 isactivated.

For example, the current limit function of the current limiter circuit 5a is enabled and disabled by applying a bias voltage to the gate of passdevice 316 (e.g., the voltage from voltage divider 304 or SOURCE COMMON300 b, via analog switch 314, is routed to the gate of the pass device316). As a particular probe 2 draws more and more current, the voltageacross its corresponding pass device 316 will rise. Eventually, the passdevice 316 will come out of saturation and enter current limiting mode,whereby current through the probe 2 is held at a steady value. If theparticular probe has a lower contact resistance than other probes, thevoltage across its respective pass device 316 will continue to rise.

The voltage across each pass device 316 is “OR'd” together via diode 320(each pass device includes a corresponding diode connected to itsdrain/collector). Thus, the pass device 316 seeing the highest voltagerelative to other pass devices will dominate, and its correspondingdiode 320 will conduct. The diodes corresponding to the other passdevices will not conduct, since, in the present example, the voltageacross these pass devices is lower.

As a result of the diode OR function, the highest voltage seen by anypass device 316 is provided to the trigger circuit 6. If that voltageexceeds a predetermined level, the SCR 7 is enabled, thereby shuntingcurrent away from the device and probes.

The current limit circuit described herein effectively provides an openloop current limiter (e.g., they operate without using a feedback signalof the controlled parameter). The system will always limit currentwhether or not current is flowing through the circuit. Operation of thecircuit is dependent on the characteristics of the pass device, and noton an active feedback loop. The open loop current limiter isadvantageous in that a feedback signal is not required, which removesthe possibility of instability in the system.

FIG. 12 is a schematic diagram of an exemplary drain (collector) currentlimiter circuit 5 b. The drain current limiter circuit 5 b is similar tothe source limiter circuit 5 a except that the relative polarities arereversed, which necessitates some re-arrangement of some of the circuitelements (e.g., the analog switch).

The drain current limiter circuit 5 b is coupled to a power source(e.g., a −10 V DRAIN REF line 330 a and a corresponding −10V DRAIN REFRETURN line 330 b). Like the source current limiter circuit 5 a above,the power may be provided via an isolated DC-DC converter. A capacitor332 or other filtering means may be coupled between the DRAIN REF line330 a and DRAIN REF RETURN line 330 b (also referred to as DRAIN COMMON330 b). Further, a voltage divider circuit 334 or the like may beconnected in parallel to the capacitor 332.

The exemplary voltage divider circuit 334 includes a first resistor 336,a variable resistor 338 and a second resistor 340. A first terminal ofthe first resistor 336 is connected to the −10V DRAIN REF 330 a, and asecond terminal of the resistor 336 is connected to a first terminal ofthe variable resistor 338. A second terminal of the variable resistor338 is connected to a first terminal of the second resistor 340, and athird terminal of the variable resistor 338, which is the voltage outputconnection of the voltage divider 334, is coupled to a first terminal ofa capacitor 342 and to a normally closed (NC) pole of analog switch 344(e.g., a single pole, double throw analog switch). The second terminalof the resistor 340 and the second terminal of the capacitor 342 areboth connected to the DRAIN REF RETURN line 330 b.

Terminals V− and GND of the analog switch 344 are connected to the −10 VDRAIN REF line 330 a, while terminals V+ and the NO pole of the analogswitch 344 are connected to the DRAIN REF RETURN line 330 b. Like theanalog switch 314 above, terminal IN of analog switch 344 controlsoperation of the analog switch. For example, by applying or removing acontrol signal at the DRAIN OFF line 345 (and thus terminal IN), thevoltage level at the COM pole of the analog switch 314 is toggledbetween the DRAIN REF RETURN line 330 b and the voltage divider output(i.e., the voltage across capacitor 342). Thus, the drain (emitter)current limiter circuits 5 a also can be individually enabled ordisabled.

Further, the common pole (COM) of the analog switch 344 is connected tothe gate of pass device 346 (e.g., a p-channel insulated gate FET),while the source of the pass device 346 is connected to the DRAIN REFRETURN line 330 b. A capacitor 348 is connected between the gate andsource of the pass device 346. The drain of the pass device 346 isconnected to the cathode of diode 350, and the anode of diode 350 isconnected to a drain limit line 352. Although not shown, the exemplarydrain current limiter circuit 5 b includes sixteen drain limit lines,each including a diode coupled thereto. The cathode of diode 350 anddrain of pass device 346 are connected to a DRAIN line 354.

With respect to the drain current limiter circuit 5 b, it is noted thata p-channel pass device is used, and that a −10V reference is providedto the circuit. Further, the analog switch is connected differently toallow operation in a reversed polarity environment, but performs thesame control function as described with respect to the source currentlimiter. Again, a diode “OR” arrangement allows the channel with thegreatest voltage drop to control the SCR crowbar trigger circuit 6 whenthe voltage across the channel exceeds the set limits. Switching thechannel on or off may be done via an isolated eight bit port asdescribed with respect to the source channels. Two ports are used tocontrol sixteen channels. Since operation of the drain current limitercircuit 5 b is similar to the source current limiter circuit 5 a,operation of the drain current limiter circuit will not be discussed forsake of brevity.

Several current limiter boards may be used in a system to provide asufficiency of channels to allow high test currents to be passed.Typically, each channel is connected to just one contacting probe, andthe control system allows any combination of source and drain channelsto be turned on.

Referring now to FIG. 13, a simulation circuit 360 for the source(emitter) and drain (collector) current limiters is shown. The circuit360 is similar to the circuit provided for unclamped inductive switchingtest in that an inductor is charged over a period of time via a fixedvoltage source. The simulation circuit 360 includes a voltage source 362connected to a series inductor 364 (together, which corresponds to theUIS tester 1 of FIG. 7) feeding four separate probe current limitercircuits 366 a-366 d. Each probe current limiter circuit 366 a-366 dincludes a MOSFET device 368, wherein a drain of each MOSFET isconnected to the inductor 364, and the gate of each MOSFET is biasedwith respect to the source of the MOSFET. Further, the source of eachMOSFET 368 a-368 d is coupled to a first terminal of a respectiveresistor 370 a-370 d, and a second terminal of each respective resistor370 a-370 d is connected to common. The resistors 370 a-370 d representtypical probe contact resistances that might be encountered during atest.

During simulation, the current in each probe current limiter circuit 366a-366 d rises in a linear manner until the voltage source is turned off,or some other element of the circuit limits the current flow. In theexemplary simulation, four MOSFETs are used as probe current limiters,each having a fixed bias supply to the gate that sets the point at whichthe device goes into constant current according to the characteristiccurves of the device. In the present example, the bias is set to 3.72volts, which sets the current limiting to about 5 Amperes. As will beappreciated by those skilled in the art, the bias and correspondingcurrent limit may vary based on the specific devices used in thesimulation. Test points 1-5 are current monitoring points used by thesimulation program for data presentation.

With further reference to FIG. 14, a graphical representation ofinductor current I_(L) in amps versus time in microseconds is shown. Itis noted that the inductor current I_(L) rises linearly untilapproximately 80 microseconds. This is the point at which the testcurrent begins to exceed the current limit of all four limiters 366a-366 d. A normal test might be set to end the current ramp up at about15 amps. Typically, the test would be configured so that it couldproceed even if one probe had excessive contact resistance. In thiscase, three limiters providing 5 amperes would allow a total current of15 amperes, even if the fourth contact was passing minimal current.

FIG. 15 is a graphical diagram showing the individual currents of eachprobe current limiter circuit 366 a-366 d. Traces 372 a-372 d representthe current in amperes for current limiter circuits 366 a-366 d,respectively. In the first current limiter circuit 366 a, the probe(represented by resistor 370 a) has the least resistance and, therefore,current rises rapidly. At about 27 μsec, the first probe current limitercircuit 366 a goes into current limit, and as the inductor current I_(L)continues to rise, the voltage across the other probe resistances willrise, causing them to carry more current (see the inflection point 374on traces 372 b-372 d). The current will continue to rise until thesecond probe current limiter circuit 372 b reaches its current limit atabout 50 μsec. At this time, the current in the remaining two probecurrent limiter circuits 372 c and 372 d rises rapidly after 50 μsec.

FIG. 16 shows the voltage across each probe current limiter circuit 366a-366 d in volts versus time in micro seconds. As can be seen in curves376 a-376 d (corresponding to circuits 366 a-366 d), once a probe hasgone into current limit mode, the voltage across the limiter rapidlyrises. This voltage is present across the other series combinations ofcurrent limiters and contact resistances. Therefore, one channel goinginto current limit will force the voltage to increase across othercontact resistances, causing them to carry more current. The currentflow is not by nature balanced for all of the probes, but better sharingof current and, more importantly, over current protection of the lowestresistance probes is achieved by this circuit arrangement.

During failure of a DUT or, in the case of having insufficient goodprobes to conduct the required test current, the voltage across all ofthe current limiters will continue to rise. Damage can occur to probesif the voltage across the contact resistance rises excessively, orworse, if any of the current limiting MOSFETs go into avalanche and losetheir ability to control current flow. To minimize this risk, thevoltage across the current limiters is monitored and, if it exceeds apredetermined safe value (e.g., about 10 volts), the crowbar triggercircuit 6 activates the SCR 7, thereby shunting the test current awayfrom the probes and current limiters, thus protecting them from damage.

FIG. 17 is a graph showing the voltage across the contact resistances involts versus time in microseconds, wherein curves 378 a-378 d correspondto voltage across probe resistors 370 a-370 d, respectively. It is notedthat as each probe current limiter 366 a-366 d goes into current limitmode, the voltage across the corresponding contact resistanceessentially stops rising and the voltage across the other (not incurrent limit mode) probe resistance rises faster.

The circuit utilized for current limiting has several advantages,including:

-   -   it is open loop and therefore cannot go into oscillation or        exhibit instability;    -   it is relatively simple;    -   it has low insertion loss since a voltage drop for a current        sensing element (e.g. a shunt resistor) is not required;    -   it allows all tests to be performed without affected the test        results;    -   it provides the opportunity to switch the current path on and        off, which can be utilized in the measurement of contact        resistance; and    -   the current limit point is sharply defined and sufficiently        stable for the purposes of probe protection (the pass device        type may be selected to have zero tempco point on the gate        transfer curve near the current where the device will be        utilized).

Moving now to FIG. 18, illustrated are logical operations to implementan exemplary method of measuring contact resistance between probes 2 andbonding pads 3. The flow chart of FIG. 18 may be thought of as depictingsteps of a method. Although FIG. 18 show a specific order of executingfunctional logic blocks, the order of executing the blocks may bechanged relative to the order shown. Also, two or more blocks shown insuccession may be executed concurrently or with partial concurrence.Certain blocks also may be omitted. In addition, any number offunctions, logical operations, commands, state variables, semaphores ormessages may be added to the logical flow for purposes of enhancedutility, accounting, performance, measurement, troubleshooting, and thelike. It is understood that all such variations are within the scope ofthe present invention.

The logical flow may begin in block 400 wherein all collector andemitter circuits are enabled. For example, the analog switches 314 and344 (FIGS. 11 and 12) are controlled via SOURCE OFF control line 315 andDRAIN OFF control line 345, respectively, such that the pass devices 316and 346 have a bias voltage applied to their respective gates. Next atblock 402, the DUT 4(FIG. 7) is turned on, and a current is injectedinto the system via power supply 212 and current source 214. Theinjected current is a fixed current preferably in the range of about 500mA to about 1 A.

At block 404, the voltage across the collector (drain) current limiters5 b and the emitter (source) current limiters 5 a is measured. At thispoint, all the emitter current limiters 5 a are in parallel, and all thecollector current limiters 5 b are in parallel (including the respectiveprobes coupled to the current limiters). Thus, the effect of eachindividual probe's resistance essentially becomes negligible, and theactual voltage measurement is the inherent voltage drop across the DUT4.

At block 406, all current limiters of a particular group (e.g., allcollector current limiters 5 b or all emitter current limiters 5 a)except one are turned off. For example, when measuring the resistance ofa single collector probe, the current limiter associated with that probeis enabled, as are all of the emitter current limiters. The remainingcollector current limiters are disabled. Then at block 408, the voltageacross the collector current limiters 5 b and the emitter currentlimiters 5 a is measured.

At block 408, the resistance for the probe is determined based on thefixed current and the inherent voltage drop across the DUT 4. Forexample, if the fixed current is 1 Amp, the first measured voltage is0.7 Volts (i.e., the inherent voltage drop across the DUT), and thesecond measured voltage is 1 volt, then the resistance of the probe is0.3 Ohms (e.g., R=1V−0.7V/1 Amp).

At block 412, it determined if the measurement is to be performed foradditional probes. If no additional measurements are to be taken, thenthe method ends. However, if more measurements are to be made, then atblock 414 the currently enabled current limiter is disabled, and thenext current limiter is enabled. Then the method moves back to block 408and repeats.

Specific embodiments of the invention have been disclosed herein. One ofordinary skill in the art will readily recognize that the invention mayhave other applications in other environments. In fact, many embodimentsand implementations are possible. The following claims are in no wayintended to limit the scope of the present invention to the specificembodiments described above. In addition, any recitation of “means for”is intended to evoke a means-plus-function reading of an element and aclaim, whereas, any elements that do not specifically use the recitation“means for”, are not intended to be read as means-plus-functionelements, even if the claim otherwise includes the word “means”.

Although the invention has been shown and described with respect to acertain preferred embodiment or embodiments, it is obvious thatequivalent alterations and modifications will occur to others skilled inthe art upon the reading and understanding of this specification and theannexed drawings. In particular regard to the various functionsperformed by the above described elements (components, assemblies,devices, compositions, etc.), the terms (including a reference to a“means”) used to describe such elements are intended to correspond,unless otherwise indicated, to any element which performs the specifiedfunction of the described element (i.e., that is functionallyequivalent), even though not structurally equivalent to the disclosedstructure which performs the function in the herein illustratedexemplary embodiment or embodiments of the invention. In addition, whilea particular feature of the invention may have been described above withrespect to only one or more of several illustrated embodiments, suchfeature may be combined with one or more other features of the otherembodiments, as may be desired and advantageous for any given orparticular application.

1. A test apparatus for applying high current test stimuli to asemiconductor device in wafer or chip form, said semiconductor deviceincluding a plurality of contact points, comprising: a plurality ofprobes for electrically coupling to respective ones of the plurality ofcontact points on the semiconductor device a plurality of currentlimiters electrically coupled to respective ones of the plurality ofprobes, said current limiters operative to limit current flow in acorresponding probe; and a current sensor electrically coupled to eachof the plurality of probes, said current sensor operative to provide asignal when detected current in any probe exceeds a predetermined level.2. The test apparatus of claim 1, further comprising an inhibit circuitoperatively coupled to said current sensor, wherein when the currentsensor generates the signal, the inhibit circuit prevents current flowthrough each of the probes.
 3. The test apparatus of claim 1, whereinthe semiconductor device is at least one of an integrated circuit or apower semiconductor.
 4. The test apparatus of claim 1, wherein saidcurrent limiters comprise a power transistor in combination with controlcircuitry, said control circuitry operative to limit the current flowthrough the power transistor.
 5. The test apparatus of claim 4, whereinsaid power transistor is a power metal oxide semiconductor field effecttransistor (MOSFET) or an insulated gate bi-polar transistor (IGBT). 6.The test apparatus of claim 1, wherein said current sensor comprises acurrent limit sensing circuit operative to detect when any one of thecurrent limiters transition into a constant current mode.
 7. The testapparatus of claim 1, further comprising a silicon controlled rectifier(SCR) device coupled between an emitter/drain terminal and acollector/source terminal of the semiconductor device, wherein saidcurrent sensor comprises a current limit sensing circuit that drives theSCR device so as to shunt current away from the emitter/drain andcollector/source of the semiconductor device.
 8. The test apparatus ofclaim 1, further comprising a verification device operative to determineif a resistance between a probe/contact interface defined between aprobe and a corresponding contact point is within predetermined limits.9. The test apparatus of claim 8, wherein said verification devicecomprises measurement circuitry to sense a voltage drop across eachindividual probe/contact interface.
 10. The test apparatus of claim 9,wherein said measurement circuitry is operative to inject a measurementcurrent through each probe and to sense a resulting voltage drop acrossthe probe/contact interface.
 11. The test apparatus of claim 1, whereineach of the plurality of current limiters is operative to be enabled ordisabled independent of other ones of the plurality of current limiters.12. The test apparatus of claim 1, wherein the current limiters areconfigured as open loop controllers.
 13. A method for applying highcurrent test stimuli to a semiconductor device in wafer or chip form,said semiconductor device including a plurality of contact points,comprising: coupling a plurality of probes to at least one of theplurality of contact points on the semiconductor device; independentlylimiting current flow through each probe of the plurality of probes to apredetermined level; and providing a signal when detected current in anyprobe exceeds a predetermined level.
 14. The method of claim 13, furthercomprising inhibiting current flow through the probes upon detection ofa fault condition.
 15. The method of claim 13, further comprisingdetermining if a resistance between a contact interface defined betweena probe and a corresponding contact point is within predeterminedlimits.
 16. The method of claim 15, wherein determining the resistanceincludes measuring a voltage drop across each individual contactinterface.
 17. The method of claim 16, wherein measuring a voltage dropincludes injecting a measurement current through each probe andmeasuring the resulting voltage drop across the contact interface. 18.The method of claim 13, wherein independently limiting current includesusing an open loop controller to limit the current.
 19. The method ofclaim 13, further comprising generating a signal when the current flowin any probe of the plurality of probes exceeds the predetermined level.20. The method of claim 19, further comprising inhibiting current flowthrough the plurality of probes upon generation of the signal. 21.(canceled)
 22. The test apparatus according to claim 1, wherein thecurrent limiters limit the current flow in each respective probe to apredetermined level.
 23. The method according to claim 13, furthercomprising protecting the probes from an over-voltage condition based ona voltage drop across the current limiters.
 24. A test apparatus forapplying high current test stimuli to a semiconductor device in wafer orchip form, said semiconductor device including a plurality of contactpoints, comprising: a plurality of probes for electrically coupling toat least one of the plurality of contact points on the semiconductordevice a plurality of current limiters electrically coupled torespective ones of the plurality of probes, said current limitersoperative to limit current flow in a corresponding probe; a voltageprotection circuit electrically coupled to said plurality of currentlimiters, said voltage protection circuit operative to protect theplurality of probes from an over-voltage condition; and a triggercircuit electrically coupled to each of the plurality of currentlimiters and to the protection circuit, said trigger circuit operativeto detect a voltage drop across the current limiters, and when thevoltage drop across any current limiter exceeds a predetermined level,the trigger circuit is operative to command the voltage protectioncircuit to enter a protection mode.
 25. The test apparatus according toclaim 24, wherein said protection circuit comprises a silicon controlledrectifier (SCR) device coupled between an emitter/drain terminal and acollector/source terminal of the test apparatus, wherein said triggercircuit comprises a voltage sensing circuit that fires the SCR device soas to shunt current away from the emitter/drain and collector/source ofthe test apparatus.